Is incident cancer in later life associated with lower incidence of dementia?

Cancer has been associated with lower risk of dementia, although methodological issues raise concerns about the validity of this association. We recruited 31,080 men aged 65-85 years who were free of cancer and dementia, and followed them for up to 22 years. We used health record linkage to identify incident cases of cancer and dementia, and split time span to investigate this association. 18,693 (60.1%) and 6897 (22.2%) participants developed cancer and dementia during follow-up. The hazard ratio (HR) of dementia associated with cancer was 1.13 (95% CI = 1.07, 1.20) and dropped to 0.85 (95% CI = 0.80, 0.91) when 449 participants who developed dementia within 2 years were excluded. The diagnosis of cancer seems to facilitate the early detection of dementia cases. Older participants who survive cancer for 2 or more years have lower risk of receiving the diagnosis of dementia over time. The factors that mediate this association remain unclear.